Influence of water on plastic deformation of olivine aggregates: 1. Diffusion creep regime

TLDR

The study investigates how water influences diffusion creep in olivine aggregates through high‑temperature creep experiments under both hydrous and anhydrous conditions. The authors performed gas‑medium creep tests on fine‑grained olivine at 100–450 MPa and 1473–1573 K, supplied water via talc dehydration, varied water fugacity, and proposed a point‑defect model linking increased silicon interstitials to faster diffusion creep. The results show that grain‑boundary diffusion dominates deformation, with strain rates about five times higher at a water fugacity of ~300 MPa compared to anhydrous conditions, and that strain rate scales roughly as water fugacity to the 0.7–1.0 power.

Abstract

The influence of water on diffusion creep of olivine aggregates was investigated by performing high‐temperature creep experiments under both hydrous and anhydrous conditions. Deformation experiments were conducted on fine‐grained samples using a gasmedium apparatus at confining pressures of 100 to 450 MPa and temperatures between 1473 and 1573 K. Water was supplied by the dehydration of talc, which occurs near 1075 K. Water fugacities of ∼85 to 520 MPa were obtained by varying the confining pressure under water‐saturated conditions. Under both hydrous and anhydrous conditions deformation was dominated by grain boundary diffusion. At a water fugacity of ∼300 MPa, samples crept ∼5 times faster than those deformed under anhydrous conditions at similar differential stresses and temperatures. Within the range of water fugacity investigated, the strain rate is proportional to water fugacity to the 0.7 to 1.0 power, assuming values for the activation volume of 0 to 20×10 −6 m 3 /mol, respectively. We propose the following point defect model to explain this water‐weakening effect: In going from an anhydrous to a hydrous environment the charge neutrality condition changes from [Fe Me • ] = 2[V Me ″] to [Fe Me • ] = [H Me ′]. As a consequence, for olivine aggregates the concentration of silicon interstitials, the rate of silicon diffusion, and therefore the rate of diffusion creep increase systematically with increasing water fugacity (i.e., OH concentration).